1. Field of the Invention
The present invention relates to a color conversion technique of digital color image data and, more particularly, to a color management system for reproducing the same color as that of a document in a scanner, color printer, and color monitor.
2. Description of the Related Art
Recently, accompanying the spread of personal computers, the technology of input devices, such as a scanner and digital still camera, and output devices, such as an inkjet printer, has been progressed. These products are widely spread in general homes as a consumer device because of its low price despite its high performance.
However, when a color document or image is printed or duplicated, as to the color reproduction, the color reproduced by an output device is not necessarily satisfactory. The main reason thereof is that while the colors dealt with in a computer are reproduced by additive color mixing of RGB, the colors dealt with in printing are reproduced by subtractive color mixing of CMYK. The additive color mixing of RGB is a method of representing a color with what is emitted from a monitor etc. itself and the subtractive process of CMYK is a method of representing a color with light reflected from a print etc. In general, in order to cope with the difference of the color reproduction method, color management using color profiles is carried out. In a color profile, color spaces of input and output devices are described. A color profile is information indispensable to color management for printing in correct colors as well as an ICC profile of a printer or inkjet sheet. In the color management, for example, color conversion processing between an input device and an output device is performed via a PCS color space. The PCS space is a “profile connection space”, indicative of a CIELAB color space etc. In the color management, for color conversion processing, a source profile for converting the color space of an input device into the PCS and a destination profile for converting the PCS into the color space of an output device are used. That is, the color management manages the color characteristics of an input device and an output device as a profile.
FIG. 8 is a block diagram showing a configuration example of a system that realizes color management of a scanner and a printer.
A color conversion device 802 receives image data from a scanner 801, which is an input device. Next, the color conversion device 802 carries out color conversion of the received image data and outputs the result to a printer 803, which is an output device. The color conversion device 802 comprises a source profile of a scanner (scanner profile) and a destination profile of a printer (printer profile). The color conversion device 802 converts the color space of scanner (device RGB) into the PCS color space using the source profile (scanner profile). Next, the color conversion device 802 converts the PCS color space into the color space of printer (for example, CMYK) using the destination profile (printer profile).
FIG. 9 is a block diagram showing a configuration example of a system that realizes color management of a scanner and a monitor.
A color conversion device 902 receives image data from a scanner 901, which is an input device. Next, the color conversion device 902 carries out color conversion of the received image data and outputs the result to a monitor 903, which is an output device. The color conversion device 902 comprises a source profile of a scanner (scanner profile) and a destination profile of a monitor (monitor profile). The color conversion device 902 converts the color space of scanner (device RGB) into the PCS color space using the source profile (scanner profile). Next, the color conversion device 902 converts the PCS color space into the color space of monitor (for example, sRGB monitor) using the destination profile (monitor profile).
Each profile described above is represented by a three-row×three-column matrix or a three-dimensional lookup table (hereinafter, referred to as 3D-LUT). When a profile cannot be represented by a three-row×three-column matrix, it is represented by a 3D-LUT. The color conversion processing 53-123201 and using a 3D-LUT is disclosed in Japanese Patent Laid-Open No. 53-123201 (1978) and Japanese Patent Laid-Open No. 2000-022973. In the tetrahedron interpolation disclosed in these patent documents, a linear interpolation operation is carried out using four pieces of grid point data.
The source profile (scanner profile) links the device RGB for a plurality of color sample charts with calorimetric values XYZ, for example, as shown in formula (1)
                              (                                                    X                                                                    Y                                                                    Z                                              )                =                              (                                                                                m                    ⁢                                                                                  ⁢                    11                                                                                        m                    ⁢                                                                                  ⁢                    12                                                                                        m                    ⁢                                                                                  ⁢                    13                                                                                                                    m                    ⁢                                                                                  ⁢                    21                                                                                        m                    ⁢                                                                                  ⁢                    22                                                                                        m                    ⁢                                                                                  ⁢                    23                                                                                                                    m                    ⁢                                                                                  ⁢                    31                                                                                        m                    ⁢                                                                                  ⁢                    32                                                                                        m                    ⁢                                                                                  ⁢                    33                                                                        )                    ⁢                      (                                                            R                                                                              G                                                                              B                                                      )                                              (        1        )            
With a device having a high linearity, it is possible to link the device RGB with the calorimetric values XYZ using the primary matrix described above. In actuality, however, the linking is performed including the n-th term of the device RGB in most cases. When the second term of the device RGB is included, the device RGB for the plurality of color sample charts and the colorimetric values XYZ are linked, for example, as shown in formula (2).
                              (                                                    X                                                                    Y                                                                    Z                                              )                =                              (                          M              ij                        )                    ⁢                      (                                                            R                                                                              G                                                                              B                                                                                                  R                    2                                                                                                                    G                    2                                                                                                                    B                    2                                                                                                RG                                                                              GB                                                                              BR                                                                              C                                                      )                    ⁢                      (                                                                                                      i                      =                                              1                        ⁢                                                                                                  ⁢                        …                        ⁢                                                                                                  ⁢                        10                                                              ,                                                                                        j                    =                                          1                      ⁢                                                                                          ⁢                      …                      ⁢                                                                                          ⁢                      3                                                                                            )                                              (        2        )            
Here, Mij is a quadratic matrix coefficient. C is a constant term and used when necessary. When the device RGB and the calorimetric values XYZ are linked using the n-order matrix, the profile is represented by a 3D-LUT in general.
In the conventional technique, a 3D-LUT is used, which performs linking using a high-order matrix. However, a matrix is defined so as to achieve total optimization, and therefore, partially, it cannot necessarily achieve an excellent color conversion precision. Because of this, a technique for further improving the color conversion precision in a specific color gamut has been proposed. For example, in Japanese Patent Laid-Open No. 2001-320594, a technique for improving the color conversion precision in a gray region has been disclosed. For this technique, there are provided a color conversion table A in which reference values are defined at predetermined grid intervals and a color conversion table B for the gray region in which reference values are defined at grid intervals smaller than the predetermined grid intervals. Then, by switching the color conversion tables in accordance with a color gamut of the input side, the color conversion precision in the gray region is improved.
In Japanese Patent Laid-Open No. 2000-253270, based on an initial color conversion table corresponding to grid points, a color conversion table is corrected by weighting based on the output values, the colorimetric values, and the distance to the grid point using a plurality of color patches in the vicinity of the grid point.
The method disclosed in Japanese Patent Laid-Open No. 2001-320594 has a problem that the color conversion processing is complicated, such as the processing of linear interpolation branches in accordance with a color gamut, because the color conversion tables are switched in accordance with the color gamut of an input device. Further, with the method disclosed in Japanese Patent Laid-Open No. 2000-253270, although the color conversion precision can be improved partially, a high-precision color conversion cannot be realized in a specific color gamut.